There are many technical processes in which it is desired to contact gas and liquid. In particular, mention may be made of dissolving oxygen, either as such or from air into water in order to aerate it, e.g. to supply oxygenated water to fish farming tanks or effluent treatment tanks. Likewise, substantial efforts have been devoted in dissolving carbon dioxide into water or aqueous mixtures in order to to make carbonated beverages. In addition to dissolving gases in liquids, gas/liquid contacting operations may be used to remove certain dissolved gases from a liquid, for example stripping dissolved oxygen in water using nitrogen.
Numerous other applications will occur to those skilled in the art and in this connection it should be observed that the term gas and liquid as used herein are to be widely interpreted. The term gas includes both gaseous mixtures of two or more gases, vapours and suspensions of liquid or solid materials in a gas or vapour. Likewise the term liquid includes mixtures of more than one liquid, whether the components are miscible or immiscible, and solutions or dispersions of gases or solids or both in a liquid. This also includes emulsions. A specific application of the process of the inventoin has been found to be in the treatment of viscous liquid triglyceride oils and fats with gaseous hydrogen in order to reduce the degree of unsaturation present in naturally occurring feedstocks. The process is particularly useful in the hydrogenation of polyunsaturated edible oils.
Many patent specifications have been published describing methods and apparatus for contacting gases and liquid. In many of these gas and liquid are fed to some form of vertical column and at least one stream, normally treated liquid, is led away from the column. Generally speaking the bulk liquid flow in the column is downward in order at least partially to counteract the upward flow of bubbles of the gas being treated due to their buoyancy.
U.S. Pat. Spec. No. 2,128,311, 3,476,366, 3,826,742 and 4,138,330 all describe apparatus for contact of gas and a liquid in which a liquid inlet stream is passed through a gas space at the top of the column to impinge on liquid on that column, i.e. waterfall effect. While such systems do operate to give contacting between gas and liquid, they tend to be inefficient in energy terms. In particular, the liquid stream loses substantial kinetic energy as it enters and passes through the gas space at the top of the column and this energy is accordingly not used for securing good contacting.
In all these and most downflow columns the dispersion of the gas into bubbles takes place in the column. Introducing the gas directly into the column or into a pipeline at normal velocities gives rise to a non-uniform bubble size, typically from 0.25 mm to 12.0 mm in diameter, with a high percentage of the larger bubbles. British Patent Specification No. 1,596,738 discloses a process in which a liquid stream is fed vertically into the top of a column at a velocity above a critical value. The process has a high energy consumption which sets up oscillating flow patterns which move first in one direction then stop and then rapidly move in another direction, sometimes reversing their direction, particularly in the lower half of the contacting zone. During these rapid changes in direction there is a tendency for pockets of bubbles to become trapped and near stationary, reducing the rate of mass transfer. In such stationary pockets liquid is enabled to drain from the bubbles which then coalesce and give rise to a very large size bubble. This rapidly rises to the top of the column and forms a layer of separated gas which leads to inefficient operation and which, if allowed to build up, precludes further operation of the device. In addition, as such large bubbles rise through the circulating froth, they promote further instabilities in the froth and the whole operation is destablilised as a result. Further, the severe oscillating nature of the bubble structure gives rise to numerous minute bubbles which are difficult to separate and can leave in the exit liquid stream.
It is therefore an object of the invention to provide an improved method of gas-liquid contacting. It is a further object of the invention to provide a method of gas-liquid contacting in which a mass of liquid and gas bubbles are formed and subjected to a rotational movement about a vertical axis and simultaneously to movement along the axis itself.